The present invention relates to techniques for drilling wells or shafts in the subsoil and more particularly to the case where the drilling cavity must at least partly be formed of zones having a certain curvature.
In this special case, it is sometimes necessary to guide the drilling tool into the curved zones by using markings obtained with the aid of magnetic measurements with respect to the direction and intensity of the geomagnetic field. When drilling takes place in highly curved areas, it is conventional practice to use drill strings comprising members in the form of solid or hollow cylindrical bars having e.g. a length of 20 to 50 m, for advancing the cutting tool into the drilling cavity. Unfortunately, the motor of the tool, as well as the drill string members used for advancing it are generally made from highly ferromagnetic materials, such as iron or ordinary steel, and consequently have a significant magnetization. This magnetization has the serious defect of interfering with the ambient magnetic field and consequently of falsifying the readings of the positioning sensor with respect to the geomagnetic field if said sensor is positioned in the vicinity of the drill string members as is necessary in practice.
Hitherto the solution adapted to overcome this problem has consisted of using non-magnetic stainless steel for making the drill string members closest to the positioning sensor. Unfortunately, this metal is very expensive so that this solution is not ideal.